I. Field of the Invention
The present invention elates to a circuit for limiting the output voltage of a monolithic semiconductor power device which drives a resonant load connected to a power supply.
II. Prior Art and Other Considerations
For driving resonant loads, such as an ignition coil or a transformer, use is made of a semiconductor power device, say, of the darlington type, which is periodically switched on and off by means of a control circuit operated by an appropriate input signal.
The power device in substance operates like a switch that is opened or closed by the input signal provided by the control circuit.
When the switch is closed, a current flows through the load. When the switch is opened, a positive overvoltage is established between the load and the switch followed by a series of negative voltage peaks which depend on the parasitic capacities of the load and on how the energy, which is stored in the load itself during the switch's closing step, is discharged. Such negative voltage peaks can bring the output voltage of the power device below ground by several volts.
In the case wherein the control circuit and the power device are accomplished in the form of an integrated circuit, parasitic components are associated with the active components of the control circuit which, when the above voltage falls below ground, enter into conduction and shortcircuit the active components of the control circuit and jeopardize the operation of the same.
The object of the present invention is to prevent the output of the power device, that is, its collector connected to the load, from going to a negative voltage with respect to ground thus triggering the parasitic components.